A Probe-Based qPCR Method, Targeting 16S rRNA Gene, for the Quantification of Paenibacillus larvae Spores in Powdered Sugar Samples

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Paenibacillus larvae (P. larvae) is responsible for American foulbrood (AFB), the most severe bacterial disease of honeybees. The enumeration of P. larvae spores in substrates taken from hives allows for the identification of the contamination levels of the colonies, mostly in those with atypical symptoms or with asymptomatic infections; in these cases, it is essential for the effective control of American foulbrood (AFB). In this work we described a new quantitative TaqMan® probe-based real-time PCR (qPCR) assay, targeting the 16S rRNA gene of P. larvae, used for the quantification of P. larvae spores in powdered sugar samples collected from hives, in comparison to the culture. A total of 105 colonies were selected, belonging to 10 apiaries with different levels of infection, located in northern Italy. The proportions of positive colonies was 54% (57/105) with the culture and 66% (69/105) with qPCR. A significant difference between the two methods was found with McNemar’s test (p = 0.02). Out of the 51 positive samples by both methods, 45 showed higher infection by qPCR compared to the culture. A close concordance with the clinical–epidemiological status of the hives was observed by both methods, with higher infection levels found by qPCR.